The present disclosure relates to fasteners for assembling two or more panels of a workpiece, and more particularly to blind fasteners for assembling relatively thin sheet material or fragile laminated composites such as in aircraft structures or hybrid structures comprising metal and composites.
Joints in an airframe are generally subjected to shear and tensile loads such that mechanical fasteners used to construct those joints must function in cooperation with structural elements of the airframe to resist deformation and overloading. It is not enough to have a failure-proof fastener because, in many instances, it is a structural element that contains a weak link that causes failure. Mechanical fasteners must therefore be compatible with the materials and structural shapes that are joined so as to maximize the joint strength at the least cost. To help resist slippage or unwanted elongation of joints, aircraft fasteners are made to impart high clamping loads and/or to completely fill the aligned apertures in the structural elements.
Laminated composites pose a special challenge because high clamping loads can cause crushing or delamination damage that can degrade the strength of the composite material. Similarly, excessive press fits between the fastener and apertures in composite panels can also result in delamination and weakening. For this reason, fasteners destined for installation into laminated composites are typically designed with close-fit shanks to fill the apertures and enlarged bearing surfaces to spread the clamping loads over a large region. Providing enlarged bearing surfaces is relatively easy with conventional fasteners such as nuts and bolts as washer may be used. However, this is more difficult in blind fasteners because the fastener component providing the bearing surface for the blind-side panel must fit through the aperture and then expand somehow without damaging the fragile material.
Present blind fasteners typically form an enlarged blind side bearing surface, or upset, by either bulbing a region of the shank component against the blind side panel (such as with pull-type blind bolts) or by bulbing a separate sleeve against a surface of the blind side panel (as with threaded-type blind bolts). A drawback to the pull-type blind bolts is that the diameter of the blind head is generally limited to about 1.25D (where “D” represents the fastener shank diameter). Pull-type blind bolts also generally impart very low residual clamp to the structure with clamp loads generally in the range of 6,000 psi based upon the shank cross-sectional area. Current threaded type blind bolts, by contrast, are generally capable of producing enlarged blind heads of up to 1.5D and imparting residual clamp loads corresponding to 30,000 psi, based upon the shank cross-sectional area. For this reason threaded blind bolts tend to be the preferred style for structural joints comprising laminated composite materials in aircraft.
Drawbacks of presently-available bulbing threaded blind bolts include a limited gripping range and considerable variation in blind head bearing surface and clamping load between maximum grip condition and minimum grip condition. Other drawbacks include the need to incorporate a plastic insert into the sleeve to facilitate bulbing at a relatively low load and the need to grind or otherwise carefully control the shape of the fustro-conical ramp on a fastener body to avoid damage to the plastic insert.
A need therefore exists for a blind fastener that produces a predictable and consistent enlarged blind-side bearing surface and clamp load throughout an extended gripping range and that avoids the need for supplemental components (such as plastic inserts) or processing (such as high cost CNC machining or grinding) to control the bulbing action.